The present invention relates to a processor apparatus and method for a process measurement signal. More particularly, the present invention relates to an improved processor for time-of-flight signals to provide an accurate indication of the location of an interface between a first medium and a second medium in a vessel.
The process and storage industries have long used various types of equipment to measure process parameters such as level, flow, temperature, etc. A number of different techniques (such as mechanical, capacitance, ultrasonic, hydrostatic, etc.) provide measurement solutions for many applications. However, many other applications remain for which no available technology can provide a solution, or which cannot provide such a solution at a reasonable cost. For many applications that could benefit from a level measurement system, currently available level measurement systems are too expensive.
In certain applications, such as high volume petroleum storage, the value of the measured materials is high enough to justify high cost level measurement systems which are required for the extreme accuracy needed. Such expensive measurement systems can include a servo tank gauging system or a frequency modulated continuous wave radar system.
Further, there are many applications that exist where the need to measure level of the product is high in order to maintain product quality, conserve resources, improve safety, etc. However, lower cost measurement systems are needed in order to allow a plant to instrument its measurements fully.
There are certain process measurement applications that demand other than conventional measurement approaches. For example, applications demanding high temperature and high pressure capabilities during level measurements must typically rely on capacitance measurement. However, conventional capacitance measurement systems are vulnerable to errors induced by changing material characteristics. Further, the inherent nature of capacitance measurement techniques prevents the use of such capacitance level measurement techniques in vessels containing more than one fluid layer.
Ultrasonic time-of-flight technology has reduced concerns regarding level indications changing as material characteristics change. However, ultrasonic level measurement sensors cannot work under high temperatures, high pressures, or in vacuums. In addition, such ultrasonic sensors have a low tolerance for acoustic noise.
One technological approach to solving these problems is the use of guided wave pulses. These pulses are transmitted down a dual probe transmission line into the stored material, and are reflected from probe impedance changes which correlate with the fluid level. Process electronics then convert the time-of-flight signals into a meaningful fluid level reading. Conventional guided wave pulse techniques are very expensive due to the nature of equipment needed to produce high-quality, short pulses and to measure the time-of-flight for such short time events. Further, such probes are not a simple construction and are expensive to produce compared to simple capacitance level probes.
Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and to time their return with very inexpensive circuits. See, for example, U.S. Pat. Nos. 5,345,471 and 5,361,070. However, this new technology alone will not permit proliferation of level measurement technology into process and storage measurement applications. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 MHz or higher, and have an average power level of about 1 nW or lower. These factors present new problems that must be overcome to transmit the pulses down a probe and back and to process and interpret the returned pulses.
First, a sensor apparatus must be provided for transmitting these low power, high frequency pulses down a probe and effecting their return. Such appropriate sensor apparatus is described in U.S. Pat. No. 5,661,251 entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT and U.S. Pat. No. 5,827,985 entitled SENSOR APPARATUS FOR PROCESS MEASUREMENT, the disclosures of which are hereby expressly incorporated by reference into the present application.
The sensor apparatus is particularly adapted for the measurement of material levels in process vessels and storage vessels, but is not limited thereto. It is understood that the sensor apparatus may be used for measurement of other process variables such as flow, composition, dielectric constant, moisture content, etc. In the specification and claims, the term "vessel" refers to pipes, chutes, bins, tanks, reservoirs or any other storage vessels. Such storage vessels may also include fuel tanks, and a host of automotive or vehicular fluid storage systems or reservoirs for engine oil, hydraulic fluids, brake fluids, wiper fluids, coolant, power steering fluid, transmission fluid, and fuel.
The present invention propagates electromagnetic energy down an inexpensive, signal conductor transmission line as an alternative to conventional coax cable or dual transmission lines. The Goubau line lends itself to applications for a level measurement sensor where an economical rod or cable probe (i.e., a one conductor instead of a twin or dual conductor approach) is desired. The single conductor approach enables not only taking advantage of new pulse generation and detection technologies, but also constructing probes in a manner similar to economical capacitance level probes.
The present invention specifically relates to a signal processor apparatus for processing and interpreting the returned pulses from the conductor. Due to the low power, broadband pulses used in accordance with the present invention, such signal processing to provide a meaningful indication of the process variable is difficult. Conventional signal processing techniques use only simple peak detection to monitor reflections of the pulses.
The present invention provides signal processing circuitry configured for measurement of the time-of-flight of very fast, guided wave pulses. Techniques used in similar processes, such as ultrasonic level measurement are vastly different from and are insufficient for detection of guided electromagnetic wave pulses due to the differences in signal characteristics. For example, ultrasonic signals are much noisier and have large dynamic ranges of about 120 dB and higher. Guided electromagnetic waves in this context are low in noise and have low dynamic ranges (less than 10:1) compared to the ultrasonic signals, and are modified by environmental impedances. The signal processor of the present invention is configured to determine an appropriate reflection pulse of these low power signals from surrounding environmental influences.
Standard electromagnetic reflection measurements are known as time domain reflectometry (TDR). TDR devices for level measurement require the measuring of the time of flight of a transit pulse and a subsequently produced reflective pulse received at the launching site of the transit pulse. This measurement is typically accomplished by determining the time interval between the maximum amplitude of the received pulse. The determination of this time interval is done by counting the interval between the transmitted pulse and the received pulse.
The present invention provides an improved signal processor for determining a valid reflective pulse signal caused by an interface of material in contact with a probe element of a sensor apparatus. The processor apparatus of the present invention is particularly useful for processing high speed, low power pulses as discussed above. In the preferred embodiment of the signal processor apparatus, processing is performed based on a digital sampling of an analog output of the reflective pulses. It is understood, however, that similar signal processing techniques can be used on the analog signal in real time.
It is well known that variations in operating conditions such as environmental variations like temperature, humidity, and pressure; power variations like voltage, current, and power; electromagnetic influences like radio frequency/microwave radiated power which creates biases on integrated circuit outputs; and other conditions such as mechanical vibration can induce undesired drifts of electronics parameters and output signals. The present invention provides a processing means and method for compensating for signal drifts caused by these operating conditions.
According to one aspect of the present invention, a method is provided for processing a time domain reflectometry (TDR) signal to generate a valid output result corresponding to a process variable in a vessel. The method includes the steps of establishing an initial boundary signal, storing the initial boundary signal and detecting a TDR signal. The method also includes the steps of determining a baseline signal by subtracting the initial boundary signal from the TDR signal, determining the reflection pulses in the baseline signal due to the process variable in the vessel, and computing the level of the process variable in the vessel. This aspect presupposes that an initial boundary signal was previously established for the vessel. An initial boundary signal is ideally established by taking a measurement in the vessel when it is empty to map extraneous reflection sources in the TDR signal. However, in operation it is often impractical to empty a vessel every time a probe is installed. This presents a problem in determining the initial boundary signal to be used in the determination of the baseline signal.
A feature of the present invention is a method of determining the boundary signal without requiring that the vessel be emptied. The process of partial probe mapping combines a background signal with a sample TDR signal to create a partial probe map that can be used as the initial boundary signal. The background signal provides an estimate for probe reflections at the end of the probe and other fluctuations in the immersed portion of the newly installed probe. The sample TDR signal provides a mapping of the reflections from vessel artifacts and other sources above the level of the material in the vessel. Partial probe mapping determines the offset to be applied to the background signal and combines the background signal for one portion of the probe and the sample TDR signal for another portion of the probe to calculate an initial boundary signal for use in determining the process variable in the vessel. The calculation of the offset to compensate for differences in the sample TDR signal and the background signal is required for the partial probe mapping.
In one aspect of the present invention a transition point is chosen on the signals to be used as the point where the signals are combined. An offset adjustment is computed to account for differences between the two signals as the difference between the sample TDR signal at the transition point and the background signal at the transition point. This assures an equal signal value for both the background signal and the sample TDR signal at the transition point eliminating any discontinuity at the transition point in the partial probe map.
In another aspect of the present invention an offset adjustment is computed to account for differences between the two signals as the difference between the average signal value of the sample TDR signal above the transition point and the background signal below the transition point. This makes use of the two portions of the signals used to create the partial probe map.
In yet another aspect of the present invention an offset adjustment is computed to account for differences between the two signals as the difference between the average value of the sample TDR below the transition point and the background signal below the transition point.
In another aspect of the present invention an offset adjustment is computed to account for differences between the two signals as the difference between the average signal value over the entire range of the sample TDR signal and the average signal value over the entire range of the background signal.
In another aspect of the present invention an offset adjustment is computed to account for differences between the two signals as the difference between the average signal value over the portion of the sample TDR signal above the transition point and the average signal value over the portion of the background signal above the transition point.
In yet another aspect of the present invention an offset adjustment is computed to account for differences between the two signals as the difference between the average signal value over a small interval of the sample TDR signal above the transition point and the average signal value over a small interval of the background signal above the transition point.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.